Switching power supply with a snubber circuit

ABSTRACT

A DC/DC power supply with two transformers ( 10, 20 ), which for example are connected in parallel on the primary and in series on the secondary side, comprises two passive, non-dissipative snubber circuits ( 1.1, 1.2 ). Each snubber circuit comprises a series circuit formed by a capacitor ( 3.1, 3.2 ) and a diode ( 4.4, 4.7 ), which is connected to the output capacitor ( 13 ) of the power supply. The transformers are interleaved with a push-pull configuration. On the secondary side, the power supply has an output choke ( 12, 22 ) and two output rectifiers ( 11, 21 ) are connected to the secondary winding ( 14, 24 ) of each transformer. Each snubber circuit further comprises two additional diodes ( 4.5, 4.6, 4.8, 4.9 ), one electrode of these diodes being connected to the joint electrode of the snubber capacitor ( 3.1, 3.2 ) and the first snubber diode ( 4.4, 4.7 ) and the other electrodes being connected respectively to an end of the secondary winding ( 14, 24 ) of a transformer.

TECHNICAL FIELD

The invention relates to a switching power supply providing a DC output,the power supply having an output capacitor, a transformer with aprimary and a secondary winding and a snubber circuit comprising asnubber capacitor and a first snubber diode. The invention furtherrelates to a snubber circuit for such a switching power supply.

PRIOR ART

The number of electrically powered devices has increased enormously overthe last decades. Accordingly, the demand for power supplies along withthe requirements which they have to meet, has also increased. A lowfailure rate and high efficiency are among the most importantrequirements for power supplies.

A widely used type of power supplies are the switching power supplieswhich typically are smaller, lighter and generate less heat than anequivalent linear power supply. To lower switching losses, to reduce therate of current or voltage rise or to limit overvoltages so calledsnubber circuits were introduced.

Since a snubber circuit typically comprises resistors, capacitors,diodes and/or inductors, they generate losses and produce heat which hasto be dissipated. Generally spoken, they lower the overall efficiency ofthe power supply.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a switching power supply ofthe kind mentioned at the beginning, particularly to provide a switchingpower supply with a lossless or a low-loss snubber circuit.

The object of the invention is achieved by the switching power supplydefined in claim 1. According to the invention, the switching. powersupply provides a DC output and comprises an output capacitor, atransformer with a primary and a secondary winding and a snubber circuitwith a diode and a capacitor. The snubber diode and the snubbercapacitor are connected in series, having a joint electrode. In otherwords, one electrode of the snubber diode, either the anode or thecathode, is connected directly to an electrode of the snubber capacitor.According to the invention, the snubber diode and the snubber capacitorform a series circuit which is connected in parallel to the outputcapacitor.

Therefore, the snubber circuit is purely passive and since it isconnected to the output capacitor of the power supply, it isnon-dissipative too, because the energy which is stored in the snubbercapacitor during a first period of time is recycled, i. e. transferred,to the output capacitor during a second period of time.

As this energy can be used to drive a load being connected to the powersupply, it does not have to be dissipated. Therefore, lower currentsand/or voltages within the power supply are necessary in order to reachthe same power level as in a known power supply. This in turn enablesthe usage of electrical and/or magnetic components with a lower nominalpower, which typically have lower losses, resulting in a power supplywith a higher efficiency and a lower EMI (electromagnetic interference).Moreover, since the load on most of the components of the power supplyis reduced, a smaller failure rate can be achieved.

Generally, a snubber circuit can comprise other components such as forexample inductors and resistors. The snubber circuit for the inventivepower supply preferably does not comprise any inductors or resistors,but additionally to the above mentioned first snubber diode a second anda third snubber diode. Both of these diodes are connected to the jointelectrode of the first snubber diode and the snubber capacitor with oneof their electrodes, i. e. either the anode or the cathode. Preferably,the same electrode of both diodes is connected to the joint electrode.The other electrodes of these diodes are connected to the ends of thesecondary winding, i. e. the second electrode of the second snubberdiode is connected to a first end of the secondary winding of the powersupply transformer and the second electrode of the third snubber diodeaccordingly is connected to the other, the second end of the secondarywinding of the transformer.

One advantage of this snubber circuit is, that it limits the overvoltageacross the output rectifier diodes, which for example are arranged as afull bridge rectifier connected to the secondary winding. That is why itis possible to make use of diodes for lower voltages. As low voltagediodes typically have a smaller voltage drop and a shorter reverserecovery time than high voltage diodes, their usage typically results ina higher efficiency and in a lower EMI.

The snubber circuit described is well suited for usage in a DC/DCconverter. However, as every DC/DC power supply easily can be convertedto an AC/DC power supply by adding an input rectifier-and connecting anAC power source, this snubber circuit can also be used with an AC/DCpower supply.

Although it is possible to use different power supply configurationssuch as flyback, forward or other converters, the power supply in afavoured implementation of the invention is a push-pull converterbecause such converters have a low output ripple current and, due to thehigh switching frequencies, the size of the transformer can be small.The push-pull converter comprises at least two power switches, which arealternately driven as known in the art. MOSFETs (metal oxidesemiconductor field effect transistor) can for example be used to switchthe input power. Nevertheless, other electronic switches can be used aswell.

In another favoured implementation of the invention the power supplycomprises two transformers. The primary windings of the transformers areconnected in parallel while the secondary windings are connected inseries. In this embodiment of the invention, the power supply comprisestwo snubber circuits as well, where the snubber circuits are connectedto the secondary windings of the transformers respectively.

By using two transformers in parallel on the primary side, the RMS (rootmean square) current in the input capacitor of the power supply, whichtypically is an electrolytic capacitor, can be reduced. Furthermore, asa result of this design, the frequency on the secondary side of thepower supply is multiplied by four. This enables to reduce the size ofthe output choke for high power applications, resulting in a smaller andlower-cost power supply.

In this embodiment of the invention, a rectifier, for example a fullbridge rectifier, is connected to the secondary winding of eachtransformer. Hence, each snubber circuit limits the overvoltage acrossthe rectifier diodes of the rectifier which is connected to the sametransformer as the snubber circuit itself.

In the power supply embodiment with two transformers, the snubbercircuits are very similar to each other and are connected to thesecondary windings of the transformers in a similar way. Both of themcomprise a snubber capacitor as well as a first, a second and a thirdsnubber diode. The two snubber circuits only differ in the polarisationof some of their components. The two snubber circuits are connected tothe output capacitor and the secondary windings of the power supply asfollows:

-   -   The anode of the first snubber diode of the first snubber        circuit is connected to the first electrode of the output        capacitor.    -   The cathode of the first snubber diode of the first snubber        circuit is connected to the first electrode of the snubber        capacitor of the first snubber circuit and the second electrode        of the snubber capacitor of the first snubber circuit is        connected to the second electrode of the output capacitor.    -   The anode of the second snubber diode of the first snubber        circuit is connected to the cathode of the first snubber diode        of the first snubber circuit and the cathode of the second        snubber diode of the first snubber circuit is connected to the        first end of the secondary winding of the first transformer.    -   The anode of the third snubber diode of the first snubber        circuit is connected to the cathode of the first snubber diode        of the first snubber circuit and the cathode of the third        snubber diode of the first snubber circuit is connected to the        second end of the secondary winding of the first transformer.    -   The cathode of the first snubber diode of the second snubber        circuit is connected to the second electrode of the output        capacitor.    -   The anode of the first snubber diode of the second snubber        circuit is connected to the first electrode of the snubber        capacitor of the second snubber circuit and the second electrode        of the snubber capacitor of the second snubber circuit is        connected to the first electrode of the output capacitor.    -   The cathode of the second snubber diode of the second snubber        circuit is connected to the anode of the first snubber diode of        the second snubber circuit and the anode of the second snubber        diode of the second snubber circuit is connected to the first        end of the secondary winding of the second transformer.    -   The cathode of the third snubber diode of the second snubber        circuit is connected to the anode of the first snubber diode of        the second snubber circuit and the anode of the third snubber        diode of the second snubber circuit is connected to the second        end of the secondary winding of the second transformer.

In a further preferred embodiment of the invention, where the powersupply comprises two transformers, both transformers are in a push-pullconfiguration. Hence, the primary winding of each transformer comprisestwo interleaved segments, where each segment is connected in series witha power switch. In order to alternately switch the segments of atransformer, the two branches of a particular transformer, which areformed by a segment and the corresponding switch, are connected inparallel.

Typically, an output filter is placed at the output of a power supply inorder to smoothen the output voltage. The above mentioned outputcapacitor is a part of such an output filter, which further typicallycomprises an inductor connected in series with the output capacitoreither ahead of or following it. Such inductors or chokes can forexample be implemented by a winding, which is looped around a magneticcore.

In a preferred embodiment of the invention, the inductor of the outputfilter is split into two inductors, one of them being positioned aheadof the output capacitor and one following it. When using the abovementioned electrode terms for the snubber components, this means thatthe first inductor is positioned between the second electrode of thesnubber capacitor of the first snubber circuit and the second electrodeof the output capacitor. The second inductor is positioned between thesecond electrode of the snubber capacitor of the second snubber circuitand the first electrode of the output capacitor.

In order to simplify the implementation of the power supply and toreduce the costs, the two output inductors can be realised by loopingthe windings of both inductors around the same magnetic core.

From the following detailed description and from the entirety of theclaims it will be clear to a person skilled in the art, that there aremore advantageous embodiments and feature combinations of the invention.

SHORT DESCRIPTION OF THE DRAWINGS

The drawings used for illustration of the examples show:

FIG. 1 illustrates a snubber circuit according to the invention in blockdiagram form, and

FIG. 2 is a power supply schematic diagram with showing a snubbercircuit according to the invention.

In general, the same objects in different drawings are given the samereference numerals.

WAYS OF CARRYING OUT THE INVENTION

FIG. 1 shows a snubber circuit 1 according to the invention. The snubbercircuit 1 comprises several terminals 2.1, 2.2, 2.3, 2.4, 2.5 which areused for connecting the snubber circuit 1 to a power supply and forconnecting the components of the snubber circuit 1 itself. The snubbercircuit comprises a capacitor 3 and three diodes 4.1, 4.2, 4.3 wherebycapacitor 3 and diode 4.1 are connected in series via terminal 2.5. Theother diodes 4.2, 4.3 are also connected to terminal 2.5 with one oftheir electrodes.

FIG. 2 shows a power supply according to the invention, which comprisestwo snubber circuits 1.1, 1.2 as shown in FIG. 1. The power supplyconverts a DC input voltage from a voltage source 18 to a DC outputvoltage which is provided across the terminals 13.1, 13.2 of an outputcapacitor 13. A load (not shown) can be connected across the outputcapacitor 13.

The DC/DC converter comprises two transformers 10 and 20, each having aprimary winding 15, 25 and a secondary winding 14, 24. The primarywindings 15, 25 are connected in parallel across the terminals of thevoltage source 18. The negative terminal of the voltage source isconnected to ground 19.

The primary windings 15, 25 of the transformers 10, 20 are interleavedand have two winding segments 15.1, 15.2 and 25.1, 25.2 respectively,whereby the winding segments of a particular transformer are connectedin parallel. So the winding segments 15.1, 15.2 of transformer 10 areconnected in parallel to ground 19 and the winding segments 25.1, 25.2of transformer 20 are connected in parallel to ground 19.

To switch the input voltage, each winding segment 15.1, 15.2, 25.1, 25.2is connected in series with a power switch 16, 17, 26, 27, which forexample are implemented by MOSFETs. The switching of the power switchesis controlled by a controller (not shown) as known in the art.

On the secondary side of the transformers 10, 20, two rectifiers 11, 21are connected across the terminals of the secondary windings 14, 24. Inthe example as shown in FIG. 2, two diode full bridges are connectedacross the terminals of the secondary windings 14, 24. Each rectifier11, 21 comprises four diodes 31, 32, 33, 34 and 35, 36, 37, 38 which arearranged in a known manner so as to form a full bridge rectifier.

The diode full bridges themselves are connected such that the secondarywindings 14, 24 of the transformers 10, 20 are connected in series.Particularly, the anodes of the diodes 33, 34 of rectifier 11 areconnected with each other as well as with the cathodes of the diodes 35and 36 of rectifier 21.

The cathodes of the diodes 31, 32 of rectifier 11 are connected witheach other as well as to a first electrode of an output choke 12, thesecond electrode of which is connected to a first electrode of theoutput capacitor 13. Correspondingly, the anodes of the diodes 37, 38 ofrectifier 21 are connected with each other as well as to a firstelectrode of an output choke 22, the second electrode of which isconnected to the second electrode of the output capacitor 13. The secondelectrode of the output capacitor 13 is connected to ground 19.

As mentioned above, the DC/DC converter comprises two snubber circuits1.1, 1.2 each being connected to the secondary winding 14, 24 of atransformer 10, 20. According to FIG. 1, the capacitor 3.1 and the firstdiode 4.4 of snubber circuit 1.1 are connected in series such that theanode of diode 4.4 is connected to the second electrode of the outputcapacitor 13, the cathode of diode 4.4 is connected to a first electrodeof capacitor 3.1 and the second electrode of capacitor 3.1 is connectedto the first electrode of the output choke 12. The anodes of the secondand third diodes 4.5, 4.6 of snubber circuit 1.1 are connected to thejoint electrode of diode 4.4 and capacitor 3.1 respectively, while theircathodes are connected to the first and the second end of the secondarywinding 14 of transformer 10.

Regarding snubber circuit 1.2, the capacitor 3.2 and its first diode 4.7are connected in series such that the cathode of diode 4.7 is connectedto the first electrode of the output capacitor 13, its anode beingconnected to a first electrode of capacitor 3.2 and the second electrodeof capacitor 3.2 being connected to the first electrode of the outputchoke 22. The cathodes of the second and third diodes 4.8, 4.9 ofsnubber circuit 1.2 are connected to the joint electrode of diode 4.7and capacitor 3.2 respectively, while their anodes are connected to thefirst and the second end of the secondary winding 24 of transformer 20.

The output chokes 12 and 22 are simply implemented by reducing them toone coupled choke.

While the rectifiers 11, 21 are described as full bridge rectifiers, itis generally also possible to use other rectifier configurations such asfor example a half bridge rectifier.

Furthermore, the usage of the invention is not restricted to thepush-pull converter topology shown in the drawings, it can also beapplied to other topologies such as for example flyback, forward orother converters. If a push-pull configuration is used, other powerswitches than MOSFETs such as for example thyristors, GTO (gateturn-off) thyristors or IGBTs (insulated gate bipolar transistor) may beappropriate.

By adding an input rectifier, the invention can of course also beapplied to AC/DC power supplies of any type.

To summarise it can be stated that the invention enables theimplementation of a lossless snubber circuit, which can be utilised in awide field of DC power supplies to reduce the failure rate as well as toimprove the overall efficiency.

Although preferred embodiments of the invention have been described indetail, it will be readily appreciated by those skilled in the art thatfurther modifications, alterations and additions to the inventionembodiments disclosed may be made without departure from the spirit andscope of the invention as set forth in the appended claims.

1. A switching power supply providing a DC output, the power supplyhaving an output capacitor, a transformer with a primary and a secondarywinding and a first snubber circuit comprising a snubber capacitor and afirst snubber diode, characterized in that the snubber capacitor and thefirst snubber diode are arranged in a series circuit which is connectedin parallel to the output capacitor, wherein the snubber circuit furthercomprises a second and a third snubber diode, a first electrode of thesecond snubber diode and a first electrode of the third snubber diodebeing connected to a joint electrode of the first snubber diode and thesnubber capacitor, a second electrode of the second snubber diode beingconnected to a first end of the secondary winding and a second electrodeof the third snubber diode being connected to a second end of thesecondary winding.
 2. The switching power supply according to claim 1,wherein the power supply has, in operation, a DC input voltage and a DCoutput voltage.
 3. The switching power supply according to claim 1,further comprising at least two alternately driven power switchesconnected to the primary winding.
 4. The switching power supplyaccording to claim 2, further comprising at least two alternately drivenpower switches connected to the primary winding.
 5. The switching powersupply according to claim 4, further comprising a second snubber circuitand a second transformer having a primary and a secondary winding, theprimary windings of the transformers being connected in parallel, thesecondary windings of the transformers being connected in series and thefirst snubber circuit being connected to the secondary winding of thefirst transformer and the second snubber circuit being connected to thesecondary winding of the second transformer.
 6. The switching powersupply according to claim 5, wherein both snubber circuits comprise asnubber capacitor and a first, a second and a third snubber diode, ananode of the first snubber diode of the first snubber circuit beingconnected to a first electrode of the output capacitor, a cathode of thefirst snubber diode of the first snubber circuit being connected to afirst electrode of the snubber capacitor of the first snubber circuitand a second electrode of the snubber capacitor of the first snubbercircuit being connected to a second electrode of the output capacitor,an anode of the second snubber diode of the first snubber circuit beingconnected to the cathode of the first snubber diode of the first snubbercircuit and a cathode of the second snubber diode of the first snubbercircuit being connected to a first end of the secondary winding of afirst transformer, an anode of the third snubber diode of the firstsnubber circuit being connected to the cathode of the first snubberdiode of the first snubber circuit and a cathode of the third snubberdiode of the first snubber circuit being connected to a second end ofthe secondary winding of the first transformer, a cathode of the firstsnubber diode of the second snubber circuit being connected to thesecond electrode of the output capacitor, an anode of the first snubberdiode of the second snubber circuit being connected to a first electrodeof the snubber capacitor of the second snubber circuit and a secondelectrode of the snubber capacitor of the second snubber circuit beingconnected to the first electrode of the output capacitor, a cathode ofthe second snubber diode of the second snubber circuit being connectedto the anode of the first snubber diode of the second snubber circuitand an anode of the second snubber diode of the second snubber circuitbeing connected to a first end of the secondary winding of a secondtransformer, a cathode of the third snubber diode of the second snubbercircuit being connected to the anode of the first snubber diode of thesecond snubber circuit and an anode of the third snubber diode of thesecond snubber circuit being connected to a second end of the secondarywinding of the second transformer.
 7. The switching power supplyaccording to claim 5, wherein the primary winding of each transformercomprises two interleaved, parallel connected segments, each segmentbeing connected in series with a power switch.
 8. The switching powersupply according to claim 7, wherein the second electrode of the snubbercapacitor of the first snubber circuit is connected to the secondelectrode of the output capacitor via a first inductor and the secondelectrode of the snubber capacitor of the second snubber circuit isconnected to the first electrode of the output capacitor via a secondinductor.
 9. In a snubber circuit having a snubber capacitor and atleast one snubber diode for a switching power supply having an outputcapacitor and providing a DC output; the improvement comprising thesnubber diode and the snubber capacitor being connected in series, inoperation the snubber circuit transferring energy stored in the snubbercapacitor to the output capacitor of the switching power supply, furthercomprising a second and a third snubber diode, a first electrode of thesecond snubber diode and a first electrode of the third snubber diodebeing connected to a joint electrode of the first snubber diode and thesnubber capacitor, a second electrode of the second snubber diode beingconnectable to a first end of a secondary winding of the power supply,and a second electrode of the third snubber diode being connectable to asecond end of the secondary winding of the power supply.
 10. Theswitching power supply according to claim 8, wherein the first andsecond inductors have a common core.